21 Materials Engineering Essay Example

MATERIAL ENGINEERING

Material Failure

Name/Number:

In this report, it was found that the cause of failure of the drive shaft was fatigue failure, caused by repeated cycles of stresses and strains. To prevent the part form fatigue failure, this report recommends an improvement in the design of the part. This report also looked at the failure of the turbine parts from a Rolls Royce T56-A15 turbo prop engine from a hurcules aircraft. Our investigation using dye penetrant testing confirmed the presence of cracks, which were caused by creep, stress corrosion and sulphidation. To prevent the failure of a turbine blade, the report recommended the use of supper alloy materials, such as a nickel-based super alloy.

Introduction

Failure can be defined as an event that that does not accomplish the intended purpose. When a material component fails, it loses its ability to function normally. There are many ways through which a material may fail, for example corrosion, excessive deformation, fracture, thermal, electrical or magnetic deformation, burning out etc. Automotive power transmission and gas turbine components sometimes encounter failures. These failures occur to some common reasons such as raw material faults (incorrect material selection), raw material processing faults, manufacturing and design faults, maintenance faults, and also user originated faults. Automobile drive shafts transmit torque, which subject the component to shear stress and torsion due to the difference between the input force and the output load. They are used to deliver transmission power to the wheels (Duffner, 2006). Thus, drive shafts require enough strength to bear the shear stress without imposing an additional inertia by virtue of its weight. Gas turbine blades also fail after sometime of service life. These components fail as a result of exposure to the high temperatures generated in the gas turbine engine which create corrosive and oxidative environment, extreme vibrations, and stress corrosion fatigues. The engineer must anticipate the potential for failure of machine parts, and exercise the best design, material selection, quality control and testing techniques (Maintenance Technology, 2012). Establishing the cause of failure of a component helps to prevent future failures by taking appropriate preventive measures.

In this study report, we investigated the types and causes of failure in the drive shaft and a turbine component. The drive shaft is from a Toyota 4×4 pick-up truck that failed at 40,000 miles having been used off road. The turbine component is a turbine blade from the T56-A15 engine that has been removed from service due to a defect. The report presents an analysis of failures of the components and predict the most likely cause of each type of failure, giving reasons and evidence to show why the predicted cause is the most likely. Finally, recommendations have been made for adjustments to the design and or manufacturing process for the drive shaft to try and prevent future failures.

Report Analysis and Discussion

The possible causes for the failure of a drive shaft from a Toyota 4×4 pick-up track. The drive shaft sheared after the vehicle had covered 40,000 miles with 1/3 of which was off-road. The normal service duty of this part is 100,000 miles from the inspection of the part which is stored in W08.

Possible causes for material failure

The general types of material failure include:

  • Failure due to fatigue

  • Failure due ductile fracture

  • Failure due to brittle fracture

  • Creep failure due to low strain rate

  • Failure by combined effects of corrosion and stress

  • Failure due to excessive component wear

  1. Fatigue failure

Fatigue failure is one of the most common causes of drive shaft failure. Fatigue in the metal shaft is caused by repeated cycling of the transmitted load (tensile, compressive, torsional or a combination of these loads). It is a progressive localized damage that results from fluctuating strains and stresses on the material. Cracks initiated by localized metal fatigue propagate in regions with more severe strain. Thus, material fatigue failure occurs in three main steps: (i) crack initiation, (ii) crack propagation, and (iii) failure (Momcˇilovic´, et al., 2012). The initial crack occurs during the crack initiation stage. Cracking may be caused by surface scratches due to tooling or handling of the material, slip bands, threads, or dislocations intersecting the material surface as a result of work hardening and repeated cyclic loading. After crack initiation, the crack continues to expand due to continuously applied stresses. Failure occurs in the final stage when the material the material part that is free of crack can no longer withstand the stress applied. Failure happens very quickly. The diagrams in figure 1 below shows some examples of components that have failed due to fatigue.

21 Materials Engineering21 Materials Engineering 121 Materials Engineering 221 Materials Engineering 3

Figure 1: (a) Failure of a fan shaft due to torsional fatigue after installation of a variable speed drive. (b) Pump shafts that have failed due to torsional fatigue leading to reduction in material strength caused by corrosion.

Material failure due to fatigue can be determined by examining the site of the fracture. There are two distinct regions in a fatigue fracture: one region is burnished or smooth due to the rubbing of the crack (from step 1 and 2), and the second one is granular due to rapid material failure (step 3).

  1. Ductile Failure

Ductile failure or ductile fracture is a type of failure that occurs in ductile or malleable materials. Ductile failure is caused by overloading these materials or subjecting them to a load at high temperatures. This causes the material to pull apart and deform instead of cracking. Ductile failure begins with a crack that grows slowly and is accompanied with plastic deformation. Under high levels of stress, the crack expands. Before ductile failure occurs, there is absorption of massive quantities of energy characterized by a slow propagation. Thus, ductile failure occurs in three basic steps: (i) void formation, (ii) void coalescence or crack formation, and (iii) failure.

Ductile metals with high purity can sustain plastic deformations of up-to 50-100%, or even more before they fracture under favourable environmental and loading condition. The strain level at which fracture occurs is controlled by the purity of the material used in manufacturing a component. In ductile fracture.

  1. Brittle Failure

Brittle failures occur in materials such as glass and ceramics and are normally caused by sudden shock loading or impacy to a material. Generally, low temperatures and high rates of deformation promote brittle fracture. The difference between failure of ductile materials and failure of brittle materials is that brittle materials fracture without undergoing plastic deformation. Brittle failure can occur very rapidly, creating a catastrophic event (Kolkman & Vleghert, 1986).

It is well understood that brittle failure and ductile failure are relative, and therefore, interchange between these two modes of material failure can easily be achieved depending on material properties. Many metal materials which become ductile at high temperatures become brittle after cooling at a temperature below the critical temperature, or the ductile-to-brittle transition temperature (DBTT) (Kolkman & Vleghert, 1986). For glass material, this temperature is called the glass transition temperature. As the temperature is reduced, there is sudden drop in impact energy over a relatively small range of temperature, below which the energy becomes of low value as transition of brittle failure. Examples of components that have undergone brittle failure are shown in figure 2.

21 Materials Engineering 421 Materials Engineering 521 Materials Engineering 621 Materials Engineering 7

Figure 2: (a) Brittle fracture shown by chevron marks the input shaft. (b) Case hardened hardened section of a pump shaft showing evidence of brittle fracture due to a single bending force.

  1. Creep failure

Creep failure is a time-dependnent deformation at constant stress and elevated temperatures. Thus, creep failure is sometimes refered to as stress rapture. Creep failure is common in material components that are exposed to high temperature conditions. However, it is worth noting that the actual stress under which a component operates determines the temperature at which creep begins. Under different conditions of temperature and stress dislocation climb, diffusion-flow or dislocation glide mechanisms may dominate creep deformation (Maintenance Technology, 2012).

  1. Corrosion, wear and stress failure

Stres corrrosion is growth of crack formation on a component due to exposure to a corrossive environment. Stress corrosion can cause sudden failure in ductile materials that have been subjected to tensile stress at elevated temperatures. Stress corrosion cracking failure is chemical specific such that some material components are likely to undergo stress corrosion cracking only to a given number of chemical environments. Material components with severe stress corossion cracking may appear bright and shiny, but are filled with microscopic cracks. A combination of certain environmental conditions and stress condition may accelerate the the rate of component failure (Duffner, 2006).

Failure due to excessive wear occurs when the surface of a component interacts with another bounding surface in the working environment. Material wear is caused by unidirectional sliding, impact loads, rolling, speed, and temperature. It causes a component to loss of dimension, loss of material and displacement of a component from its original position. The final result is failure of a component. Some examples of components that have failed due to corrosion, wear and stress failure are shown below (Du, et al., 2016).

21 Materials Engineering 821 Materials Engineering 921 Materials Engineering 1021 Materials Engineering 11

Figure 3: (a) A failed motor shaft due to badly worn out sheaves on the belt drive, (b) A slow-growing failure initiated by corrossion on a steel-mill elevator that has reduced the fatigiue strength of the shaft.

Possible cause of failure of the drive shaft

21 Materials Engineering 1221 Materials Engineering 1321 Materials Engineering 1421 Materials Engineering 15

Rubbed smooth surface

Granular surface after breaking

21 Materials Engineering 16

Figure 4: The drive shaft from a Toyota 4×4.

It is possible to determine the likely cause of failure of the drive shaft shown in figure 1 above by examining the fractiure on the surfaces of the failed parts. Based on the evidence on the fractured surfaces, it is clear from figure 4 that the likely cause fo failure of the drive shaft is fatigue . This is evident from the smooth surface on the flange due to rubbing at the crack. The second part has a granular surface due to the failure of the component. These visual clues can be observed in figure 4.

21 Materials Engineering 1721 Materials Engineering 1821 Materials Engineering 1921 Materials Engineering 20

Origin of fracture

Clamshell marking

21 Materials Engineering 21

Figure 5: The surface of fartigue fracture

Further examination of the features on the surface of the cracks reveal clamshell marks or beachmarks (see figure 5). Generally, fatigue is a common cause of shaft failure. The sharp protrutionon the shaft indicates the point of sudden failure. A fructure developed due to fatigue progresses due to repeated cycles of stresses and strains as the load is transmitted through the shaft.

Here, we examined the a part of a part of a gas turbine blade which was removed from service after its inspection and finding that it was defective due to a small crack. We carried out a dye penetration test to confirm the the type of failure and the likely causes of failure.

Methods for investigating materials failure and for estimating product service life, when a product is subject to creep and fatigue loading

  1. Visual examination

Visual examination involves visual inspection of the crack surface for fatigue cracks initiations using naked eyes.The zone of stable crack propagation front and the final instable crack growth are observed to note any marks of fatigue – such as tatchet marks, clamshel markings, dimensional changes, surface roughness and beachmarks. Physical observation of the fructured surface can provide a clue of the cause of failure, and the service life estimation of a the part.

  1. Dye penetrant test

This is one of the most popular non-destructive examination mwthods used to locate cracking defects on the surface of non-porous materials such as metals. The test involves five main steps: pre-cleaning of the part to be tested, application of the penetrant dye, removal of the penetrant, applying developer, and evaluating andications of surface defects.

  1. Magnetic Particle Inspection

Magnetic particle inspection is a very efficient method used to detect both surface and near-surface discontinuities in ferromagnetic materials. This technique relies on the principle that magnetic field spreads uniformly through a ferromagnetic material unless disrrupted by the presence of a flaw. The presence of flaw generates a locally stronger field called leakage field. The leakage field strongly attracts finely divided magnetic particles that may be in the form of dry powder or ink, which are applied to the part. If a dfect is present, an indication is produced, making the defect visible.

21 Materials Engineering 22

Figure: The principle of magnetic particle inspection

Estimate the lifetime of a component subjected to creep and fatigue loading

The lifetime of a part subjected to creep and fatigue can be estimated using methods such as multiaxial modelling that correlates fatigue life and loading parameters, Manson-Coffin equation and Larson-Miller model.

A multiaxial modelling technique – This method can be used to estimate the fatigue life of a component by correlating the fatigue lives of parts subjected to uniaxial loading using the strain-life approach (Mitchell, 1992).

Manson-Coffin equation — This equation is used to prdict component life under fatigue. Mansion Coffin recognize the fact that cyclic strain and the number of cycles are correlated (Reyhani, et al., 2013).

21 Materials Engineering 23

Where
21 Materials Engineering 24 is the number of cycles before failure,
21 Materials Engineering 25 is the material’s elastic modullus,
21 Materials Engineering 26 is fatigue ductility coefficient,
21 Materials Engineering 27 is fatigue strength coefficient,
21 Materials Engineering 28 is fatigue strength exponent, and
21 Materials Engineering 29 is fatigue ductility.

Larson-Miller model – This model is used to estimate the service life of a material under creep. Creep damage remains to be the greatest failure mechanism for turbine blades as they are exposed to high stress and high operating temperatures (Reyhani, et al., 2013). Larson describes creep using the Larson-Miller parameter which is estimated by the equation:

21 Materials Engineering 30

Where T is temperature (K),
21 Materials Engineering 31 is time to creep rupture (hours), and C is a coefficient = 20.

In the dye penetrant test, we cleaned the turbine blade part before applying the dye. The part was left for 15 minutes before spraying a developer on it, and then observing the cracks on the part.

21 Materials Engineering 3221 Materials Engineering 3321 Materials Engineering 34

21 Materials Engineering 35

Figure: Pictures of die penetrant testing procedure: precleaning, application of the die, removal of the die, and applying the developer.

The test observation revealed cracks on the part as shown in the figures below.

21 Materials Engineering 3621 Materials Engineering 3721 Materials Engineering 38

Indications of discontinuity (cracks)

21 Materials Engineering 3921 Materials Engineering 40

Figure: Pictures showing surface deffects on the surgace of the turbine blade component shown by indications of discontinuity.

There are a number of factors that can cause cracking on the surface of turbine blade part. These include: creep, stress corrosion, sulphidation, and the excessive heat produced in the turbine (thermal failure).

Creep — When a component is exposed to elevated temperatures for a longer period of time, there are changes that may occur in the component. Change within the component due to exposure to high temperatures at constant creep is called creep. For example, a turbine blade in a jet engine, gas turbine or a steam generator. These equipment operate at very high temperatures and high stresses without changes in dimensions (Kostochkin, 1962).

Stress corroison – Stress corrosion is caused by the effects of exposure of a material part to aggressive environment and increased tensile stresses. Corrosion fatigue is caused by exposure to aggressive environment and cyclic stresses. Details of this mode of failure have been discussed in the prevous section (Ziegler, et al., 2013).

Sulphidation – As the operation hours of an engine continue to accumulate, turbine vanes and blades are subject to gradual deteroriation. This process is known as sulfidation. Sulfidation in the turbine is caused by increased oxidation of metal parts due to the presence of sulfur ions — sulfates and sulfides. The oxidized metal part goes out the tail pipe, while the vane and blade are left eroded.

Thermal fatigue failure – When the turbine blade is subjected to extreme temperatures, the blade material would start to deform with time. In turbine engines, temperatures go as high as 1500˚C, which exerts a grate stress on the moving components. Turbine blades move with a speed range of 8000-15,000 RPM, and addition of thermal degradation will lead to accelerated deformation if the metal material used in making the blades is not stiff enough (Nurbanasari & Abdurrachim, 2014).

The cyclic flow of gas at varying temperatures decreases the rupture strength of the blades. A decrease of the time-to-rupture is attributed to the fact that the accumulated intergranular microdamage grows into cracks along the boundaries of the grains, thus, diminishing the carying section of the turbine blade.

A closer observation of the part examined using dye penetrant test reavealed leading and small trailing cracks. Cracks in a turbine blade are likely to result from creep, stress corrosion and sulphidation. Failure of the blade has been caused by cyclic loads (stress), aggressive environment and excessisive thermal exposure.

Conclusion

In conclusion, fatigue failure of the drive shaft is most likely due to repeated cycles of stresses and strains on the shaft. This is becase of the rubbed smooth surface and clamshell marking on the surface of the failed parts. If it was brittle or ductile failure, the parts would have chevron marks. It cannot be tortional fatigue because the parts don’t show sharp twisted ends. The failure occurred at the point wahere the shaft and the flange are attached, because that is the point where there is maximum stress and strain force.
The turbine balde most likely failed due to creep, stress corrosion and sulphidation. Thie report believes this because of the cracks observed on the turbine blade. These cracks resulted from these processes. However, without knowing the exact service life of the part however it is impossible to be more specific.

Recommendations

Preventing failure of drive shaft :

Improvenent in the design remains to be the best effective method of improving a component’s fatigue performance. This can be achived through the following:

  • Avoiding sharp surface tears that can result from shearing, stamping, punching, and other processes.

  • Eliminating or reducing stress raisers in the material by streamlining the component.

  • Elimination or reduction of tensile residual stress caused by the process of part manufacturing.

  • Improving on the details of metal fabrication and procedures used in fastening.

  • Preventing the development of discontinuities in the material during the processing.

Preventing failure of turbine blade:

In addition to the design considerations above, failure of turbine blade can be reduced by using revolutionary materials refered to as super alloys. These materials have characteristically engineered molecular structure to tolerate high thermal creep and the resulting deformations. An example of a super alloy is nickel-based super alloy -which can operate well in temperatures as high as 1000˚ without indication of deformations.

Another effective method that can be used to reduce failure of turbine blade is by coating the blade with a substance that can prevent the surface from excessive heat damage (thermal barrier coating) and corrosive environment in the turbine section.

References

Duffner, D. H., 2006. Torsion fatigue failure of bus drive shafts. Journal of Failure Analysis and Prevention, 6(6), p. 75–82.

Du, J., Liang, J. & Zhang, L., 2016. Research on the failure of the induced draft fan’s shaft in a power boiler. Case Studies in Engineering Failure Analysis, Volume 5-6, pp. 51-58.

Kolkman, H. & Vleghert, J., 1986. Fatigue failure of jet engine drive shafts. International Journal of Fatigue, 8(1), pp. 3-8.

Kostochkin, Y. V., 1962. Effect of thermal fatigue on the failure of turbine blades. Metal Science and Heat Treatment of Metals, 4(7-8), p. 305–307.

Maintenance Technology, 2012. Maintenance Technology. Available at:
[Online] http://www.maintenancetechnology.com/2012/07/failure-analysis-of-machine-shafts/
[Accessed 22 June 2017].

Mitchell, M. R., 1992. Advances in Fatigue Lifetime Predictive Techniques. New York: ASTM International.

Momcˇilovic´, D., Odanovic´, Z., Mitrovic´, R. & Atanasovska, I., 2012. Failure analysis of hydraulic turbine shaft. Engineering Failure Analysis, Volume 20, p. 54–66.

Nurbanasari, M. & Abdurrachim, 2014. Crack of a first stage blade in a steam turbine. Case Studies in Engineering Failure Analysis, 2(2), pp. 54-60.

Reyhani, M. R., Alizadeh, M. & Fathi, A., 2013. Turbine blade temperature calculation and life estimation — a sensitivity analysis. Propulsion and Power Research, 2(2), pp. 148-161.

Ziegler, D., Puccinelli, M. & Picasso, A., 2013. Investigation of turbine blade failure in a thermal power plant. Case Studies in Engineering Failure Analysis, 1(3), pp. 192-199.